A previous study (Ding et al., 2003) showed that the homeodomain transcription factor DRG11 is necessary for pattern formation in the trigeminal nucleus principalis (PrV), the requisite brainstem nucleus for development of the whisker-to-barrel cortex pathway. However, it is not known how DRG11 contributes to pattern formation. Anatomical studies were performed in DRG11 knock-out (_/_) and DRG11/Bax double_/_mice to test the hypotheses thatDRG11is required for neuronal survival in theVpathway and that PrV cell death is sufficient to explain pattern alterations. At birth, DRG11_/_ mice had equivalent cell loss in the V ganglion, PrV, and spinal V subnucleus interpolaris (SpVi). Because whisker-related patterns were normal in the SpVi, cell death would not appear to explain failed pattern formation in the mutant PrV. Electron microscopy revealed exuberant apoptosis and necrosis as the mechanisms of PrV cell death occurring in the late prenatal and newborn DRG11_/_, when such cell death was up to six times more prevalent than normal. DRG11 heterozygote and Bax_/_ mice were crossed in an attempt to dissociate PrV patterning anomalies from exuberant apoptosis in DRG11_/_ mice. Both DRG11_/_ and DRG11/Bax double _/_ mutants lacked whisker-related patterning in their PrV, despite Bax_/_-induced rescue of V ganglion and PrV cells. Thus, apoptotic cell death is not a sufficient cause of failed pattern formation in the PrV of the DRG11_/_. A signaling pathway involving DRG11 may, therefore, be the elusive PrV pattern maker. [ABSTRACT FROM AUTHOR]